The technical field of this invention is that of nondestructive materials characterization. This includes inspection of materials for hidden objects and characterization of surface, near-surface, and bulk material condition for flat and curved parts or components using magnetic field based or eddy-current sensors. A specific inspection application is the need for detection of hidden damage in complex and thick structures. This specifically includes detection of deeply buried cracks under fasteners, but also related applications that require imaging of complex constructs, such as hidden corrosion-fatigue (e.g., fatigue cracks initiating from buried corrosion damage), or hidden corrosion in difficult to inspect locations such as the F-15 wing or C-130 longeron. Another application is the detection and identification of hidden objects such as unexploded ordnance, where false indications from harmless clutter need to be suppressed in order for site remediation efforts to be effective.
The capability of existing technologies to discriminate between harmful unexploded ordnance (UXO) and harmless clutter continues to limit the efficacy of UXO remediation. These remediation sites include live ammunition testing ranges, mine fields, and areas of past military actions. Inductive sensors with a single sensing coil are commonly used for detection. These single coil methods often detect all relevant metal objects but generally cannot discriminate between UXO and harmless clutter. Also, research into model based methods, using such single coil sensors, to discriminate between UXO and clutter have shown limited success when variations in the orientation, material types, and damage are considered, even in the absence of clutter. False indications continue to far outnumber verified detections so that site reclamation is typically a slow and costly process.
According to the United Nations, there are over 100 million land mines currently deployed in more than 60 countries. The mines themselves range from large anti-tank mines to small anti-personnel mines and from all metal construction to primarily plastic or even wood. Triggering mechanisms range from direct pressure, to trip wires, to magnetic sensors and fiber optics. In addition, millions of bomblets have been deployed as Cluster Bomb Units (CBUs) during wars and military actions. A significant number of these failed to explode and continue to threaten the populations indigenous to the original combat zones. Being largely constructed of metal, unexploded bomblets are readily detectable with existing hand-held metal detectors. However, current metal detectors have no way of discriminating an intact bomblet, which may be buried at depths up to 1 m (39 in.), from a bomblet fragment or other pieces of shrapnel or metallic debris that is near the surface.
The US Army currently has a deployed mine detector called the AN/PSS-12. This is an inductive type detector that utilizes the creation of eddy currents in a metallic object to alter the impedance of the search coil. This detector has served the Army well, but to be reliably detected the object or mine must be directly beneath the search head and must contain some metal. Other methods such as ground penetrating radar, infrared, acoustic, thermal, and x-ray, have been investigated to solve the problem of detecting low-metal and no-metal mines.